A control apparatus of this kind for motor vehicle applications is described in German Patent No. 197 20 191. This control apparatus encompasses a switch unit in the form of a relay that is connectable in series with a load, for example a motor vehicle starter, a windshield wiper, etc., in order to permit delivery of energy to the load when the switch unit is switched on, and to block it when it is shut off; an electronic control system in the form of a microcontroller, for triggering the switch unit; and a delay element that is disposed in a switching path between an output terminal of the electronic control system and an input terminal of the switch unit.
During the process of starting a motor vehicle's engine, voltage dips occur in the vehicle voltage system that can make an electronic control system temporarily inoperable or, if the electronic control system is a microprocessor or microcontroller, cause it to reset. This can cause incorrect control of a load that is being controlled by way of the electronic control system. This is particularly troublesome if the load is the starter motor of a motor vehicle, since a voltage dip of this kind during the engine starting operation causes the starter motor to shut off, the starting operation is thereby terminated, and it is not possible to start the vehicle.
Although it would be possible in principle to buffer the power supply of the electronic control system in order to prevent it from entering into an undefined state during starting, a large and correspondingly expensive capacitor is necessary for the purpose, which is why it is proposed in the aforesaid document to buffer not the power supply of the electronic control system, but instead the status of the input terminal of the switch unit, by inserting the aforementioned delay element between that terminal and the output terminal of the electronic control system.
While this delay element allows short-duration level changes to be intercepted at the output terminal of the electronic control system, it also delays the switch unit's reaction to a switch-on or shutoff signal deliberately supplied by the electronic control system.